Modern lithography systems react very sensitively to heat entering their optical elements, for example lenses or mirrors, because their optical surfaces are deformed as a result of the coefficients of thermal expansion having a value other than zero of the lens materials or mirror materials used. Furthermore, the refractive index of optical elements that are used in transmission, for example lenses or plane plates, is temperature-dependent.
In a lithography system, for example in a wafer scanner for the microstructuring of semiconductor components, various local heat sources and heat sinks are present. The heat sources include absorbed radiation, in particular at the used wavelength of the lithography system, which as a result of scattering or diffraction is incident on parts of the optical system or on components arranged in the lithography system having no optical effect. In addition, in such a system there are actuators and sensors, which, depending on the type, give off heat to the environment continuously or time-dependently. In particular if the actuators are used for a movement or deformation of the optical elements, they are generally positioned very close to the optical elements.
In the case of immersion systems, i.e. in the case of lithography systems in which an immersion fluid is introduced between the last optical element of a projection lens and the substrate (wafer) to be exposed, there are in addition local cooling effects if optical elements or other components that are arranged near the immersion fluid are partially wetted with the immersion fluid and the immersion fluid on the wetted surface of the optical element or component evaporates and extracts heat from it.
In order to prevent the wetting with an immersion fluid of an unpolished surface of a component mounted outside an optically free diameter, it is known from U.S. Pat. No. 8,279,402 B2 to apply a hydrophobic coating to the unpolished surface. The hydrophobic coating may have a UV-resistant layer, which is configured to absorb or reflect UV radiation with a wavelength of less than 260 nm.
In WO 2013/087300 A1 there is a description of an optical assembly which includes at least one component that is at least partially wetted with water as an immersion fluid during the operation of the optical assembly. The component has at least one layer that contains a material of which the contact angle with the immersion fluid can be optionally set to be hydrophilic or hydrophobic. The material may be graphene and/or Co3O4 nanorod arrays.
In DE 10 2011 088 623 A1 there is a description of an optical element with an optically used volume region for the transmission of UV radiation that has a portion with a reduced thermal conductivity in comparison with the optically used volume region of the main body in order to protect the optically used volume region from thermal influences when the optical element is wetted with a fluid.
In US 2010/0279232 A1 there is a description of a microlithographic projection exposure apparatus which has a surface that is in contact with an immersion fluid during operation. The surface has a surface roughness Ra of less than or equal to 0.2 μm. The immersion fluid may have at the surface a contact angle of 60° or more. The surface may be formed on a last optical element of a projection system, a holder for a substrate or a table for a substrate. The surface may have a coating which is formed from SiOxCy.
US 2007/0146679 A1 describes a lithography apparatus which has a substrate carrier that is designed to keep a substrate in position via an electrostatic force and has an integral power source. For generating the electrostatic force, a potential difference is produced on the substrate carrier between a metallic layer and a layer of diamond-like carbon.
It is known from US 2008/0138631 A1 to provide a component, for example an optical component, for immersion lithography with a protective coating including a layer that has a hardness greater than the hardness of quartz, at least in a region that comes into contact with the immersion fluid. The layer may contain silicon carbide, diamond, diamond-like carbon, boron nitride, tungsten carbide, aluminum oxide, sapphire, titanium nitride, titanium carbonitride, titanium aluminum nitride or titanium carbide. The layer may be formed by CVD, PECVD, APCVD, LPCVD, LECVD, PVD, thin-film evaporation, sputtering or thermal annealing in the presence of a gas.
EP 2 278 402 A2 has disclosed an optical element for immersion lithography that has a light-shielding layer which is formed on a side surface of the transmissive optical element on the side of a projection system that is facing the substrate. The transmissive optical element with the light-shielding layer is in contact with the immersion fluid during the exposure. The light-shielding layer may be formed for example from a metal or a metal oxide.